If a file that has already been uploaded is modified, Backblaze will upload the delta. The part that was changed.
However, If the file is over 30MB - the file is broken into 10MB chunks to be uploaded. If the file simply has data added to the end of the file, only the last chunk will be re-uploaded. If data is entered into the middle of the file, every chunk after that point will need to be re-uploaded, because the data has been pushed back and each following block changed.